1. Field of the Invention
The present invention relates to an epoxy resin composition. More particularly, the present invention relates to an epoxy resin composition which is superior in storage stability, workability, heat resistance and film formability in B-stage, and accordingly used suitably in printed wiring board, prepreg, etc.; as well as to a prepreg or a metal foil having a resin film thereon, both produced using the composition.
2. Description of the Prior Art
In recent years, electric or electronic appliances have become lighter and thinner. For example, printed wiring boards have come to employ higher-density mounting, and various processes for production of such a printed wiring board have been developed. In particular, the process for producing a multi-layered printed wiring board using a metal foil having a resin film thereon is expected to become more popular for the convenience.
The outline of the process for producing a multi-layered printed wiring board using a metal foil having a resin film thereon is as follows. First, on the surface of an inner-layer substrate on which a circuit pattern has been formed, is placed a metal foil having thereon a resin film formed by coating an insulating resin (mainly epoxy resin) varnish and then converting the varnish into a B-stage; then, on the outer-layer metal foil is formed a circuit pattern; thereon is placed a different metal foil having a resin film of B-stage; as necessary, electric conduction is achieved between the circuit layers. By employing such a production process, it has become possible to produce a printed wiring board of high density and lightweightness.
In the midst of technical developments associated with the above-mentioned movement of electric or electronic appliances into smaller weight and smaller thickness, severe requirements are being demanded also for the insulating resins used in parts of electric or electronic appliances such as printed wiring board and the like. However, no insulating resin satisfying such severe requirements has been developed yet. That is, for example, the B-stage resin of a metal film having a B-stage resin film thereon has problems of, for example, cracking or peeling when an external force is applied thereto in winding-up by roll, etc., or scattering of resin powder when cut. Any of these problems leads to reduction in reliability of printed wiring board and must be solved.
Copper-clad laminated boards used in production of printed wiring board are produced, for example, by impregnating a substrate (e.g. glass cloth) with an insulating resin (e.g. epoxy resin) varnish, converting the varnish into a B-stage to form a prepreg, laminating a required number of such prepregs, laminating thereon a metal foil (e.g. copper foil), and subjecting the resulting material to molding under heat and pressure. In this case, there has been a problem in that the B-stage prepreg has poor handleability in the molding under heat and pressure, owing to the properties of the insulating resin (e.g. epoxy resin) and that when an external force is applied to the prepreg, there occurs the cracking or peeling of the resin.
Moreover, the resin used as an insulating resin in parts, etc. of electric or electronic appliances has insufficient heat resistance, and improvement of the heat resistance thereof has been an urgent task.
In order to solve the above problem of poor handleability in the B-stage, it has been conducted to, for example, add a resin of large epoxy equivalent to the main component resin (a resin of small epoxy equivalent). With this approach, the handleability in the B-stage is improved slightly but reduction in heat resistance is invited. In order to obtain increased heat resistance, it was also attempted to, for example, add a polyfunctional epoxy resin to the main component resin; with this approach, however, the handleability in the B-stage is inferior.
Meanwhile, addition of a polycarbodiimide (heat-resistant polymer) to an epoxy resin was proposed in order to increase the heat resistance of the epoxy resin. Application of the resulting material to a heat-resistant adhesive or the like is expected. This material, however, has a problem in that when the material is used in combination with an amine type curing agent for epoxy resin, a shorter pot life is invited.
In JP-A-9-241353 is disclosed a polycarbodiimide which was grafted and modified with trimellitic acid anhydride (a curing agent for epoxy resin). In order for an epoxy resin composition containing the above polycarbodiimide to exhibit good properties, a high curing temperature must be used; moreover, the epoxy resin composition is insufficient in gelling time of resin and a longer gelling time is required.